Bars referred to as Roebel bars, as known for example from the document EP-A1-0 778 648, are used as electrical conductors of the stator winding of rotating electrical machines, in particular turbogenerators. In the case of such a Roebel bar, a distinction is made between the straight slot part, which is in direct contact with the iron sheet of the stator, and the bent yoke part, which protrudes from the stator and by means of which each bar is connected to other bars to form a winding (see FIG. 1 of the aforementioned document). Roebel bars are provided with an insulation, in order to avoid a short-circuit with the iron sheet of the stator (slot part) or with other Roebel bars (yoke part).
As corona shielding, the surface of this insulation is usually provided with a conductive layer (sheet conductivity approximately 1000 ohms.cm/cm) in the (straight) slot part and with a semiconductive layer, which preferably has a field-dependent conductivity (sheet conductivity 107-1012 ohms.cm/cm), in the (bent) yoke part. Correspondingly, the two corona shielding layers are referred to as slot corona shielding (SCS) and yoke corona shielding (YCS).
Probably the most critical point of the SCS/YCS corona shielding system is the transition between SCS and YCS. As a result of the low resistance of the SCS, the shielding is at ground potential up to its end. In the case of high-impedance YCS, on the other hand, the capacitor coupling via the insulation gains in significance. This has the effect that, with increasing distance from the SCS end, the surface potential of the Roebel bar assumes the potential of the electrical conductor (UL). Therefore, equalizing currents flow on the surface, and these currents are particularly high in the region of the SCS/YCS transition.
This can be easily explained for the case where R(YCS)=∞. In this case, the surface potential U(x) at the point x on the yoke is given by the ratio of surface capacitance (Co) to capacitance of the insulation (Ciso). It is:U(x)=UL*ƒ(Co, Ciso)=UL*Ciso/(Co+Ciso);where x denotes the distance from the end of the SCS, which is at x=0. Co is proportional to 1/x, while Ciso is proportional to ε/d (d=insulation thickness), as in the case of a conventional plate capacitor. FIGS. 1 and 2 show the variation of U(x)/UL in arbitrary units of x, where the ratio of Co(x=1)/Ciso=0.1 (“normal coupling”, curve b) or 0.2 (“strong coupling”, curve a) is chosen. The two figures differ by different scaling of the x axis.
In the case of the field-dependent resistance of the YCS, the situation is similar, but more complicated to describe (see in this respect J. Thienpont, T. H. Sie “Suppression of surface discharges in the stator winding of high voltage machines” in “Conference Internationale des grands résaux électriques à haute tension”, Paris, 1964).
The electric field E=dU/dx is the strongest at x=0. However, this also means that there the density of the displacement currents, j, also increases, and consequently also the dielectric heat losses, which are proportional to E*j. This is the cause of the increased temperatures at the SCS end. The electric field strength itself is also problematic: it can increase so much that surface discharges occur. On the other hand, although the corona shielding helps, it does not help up to any level of voltage and capacitor coupling. Temperature increases and discharges may occur in particular under the following conditions:                When testing with Utest above 2Un for Un above 20 kV.        When testing with insulations with increased dielectric coupling (i.e. higher Ciso). Such insulation is concerned if, for example, an insulation with En≧3.5 kV/mm is to be achieved (the standard is En=2.5 kV/mm-3.0 kV/mm). For Un=.const., increasing En means a reduction in the insulating thickness by 25%-40% and a corresponding increase in Ciso. In addition to this there is also an increase in ε of about 50%, since the proportion of the mica with ε=9 is higher than in the standard insulation.        